The invention relates to an electrical switching device for switching off an overcurrent in a current path. In particular, the invention in this case relates to the field of domestic electrical power supply, to the field of relatively small and medium-size electric motors, to building technology, to lighting systems and to electrical systems in railroad vehicles, ships and the like. These fields of application can be characterized by the fact that the voltages to be switched off are typically 100 V-1 kV, and the typical load currents are in the range 0.1 A-75 A, although these numerical figures should not be regarded as representing any restriction. In particular, very large current values can occur briefly in the event of large overcurrents caused by a short circuit or the like.
The invention relates to a switching device which can switch off not only relatively small overcurrents, which are in the range 1.1 to 10 times the maximum permissible current, but also very large overcurrents of many times the maximum permissible current, in order to protect an electrical device against damage or to be prevent damage to the environment or to personnel. In the prior art, combinations of electromagnetic contactors, fuse links, thermal overload relays with a bimetallic strip as the tripping element, and the like have been used until now.
On the other hand, electrical systems and devices have been becoming ever more complex in recent times, particularly in the low-voltage field, while, on the other hand, there has been an increasing requirement to reduce the total price, the physical volume, the weight and also the power losses.
The invention is based on the technical problem of finding an improved electrical switching device for switching off overcurrents.
The invention solves this problem by means of an electrical switching device having a microrelay switch in a current path, having a short-circuit current limiter in the current path for interrupting a large overcurrent, and having an evaluation device for receiving and evaluating signals from a current sensor which detects the current through the current path, with the switching device being designed such that the microrelay switch opens in response to a tripping signal from the evaluation device in the event of small overcurrents above a threshold value and, in the event of large overcurrents, the short-circuit current limiter limits these currents to currents which can be interrupted by the microrelay switch.
The invention is furthermore also based on an electric motor switching and protection system having such an improved switching device.
The fundamental idea of the invention is thus to use a microrelay switch in conjunction with a further component for short-circuit current limiting, instead of the conventional electromagnetic contactor. In this case, the microrelay switch is intended to be designed to switch off small overcurrents, and furthermore, can also be used for the normal switching on and off of the current in the current path during normal operation. In contrast to this, a further apparatus for short-circuit current limiting is designed specifically for switching off very large overcurrents, which would destroy the microrelay switch. An evaluation device is used for detecting overcurrents and for tripping the microrelay switch but, when required, can also be actuated directly for switching on and off. The invention thus makes it possible to save the conventional components comprising thermal relays with bimetallic strips and electromagnetic contactors, and/or to replace them by a comparatively small and light electromechanical system.
The microrelay switch as such is prior art and is an electrically operated miniature switch. In contrast to a transistor, a microrelay is, however, a mechanical switch with at least one moving contact piece.
This contact piece can be caused to move mechanically by means of an electrical signal in various ways. In general, such microrelay switches are produced using known methods for microelectronics and microsystem engineering.
The devices for the invention are preferably electrostatically operated microrelay cells, that is to say those in which the moving contact piece is operated electrostatically. In this context, reference is made to the exemplary embodiments and to an Si microrelay published by Siemens (H. F. Schlaak, F. Arndt, J. Schimkat, M. Hanke, Proc. Micro System Technology 96, 1996, pages 463-468). Reference is also made to R. Allen: xe2x80x9cSimplified Process is Used to Make Micromachined FET-like Four-Terminal Microswitches and Microrelaysxe2x80x9d in Electronic Design, 8 Jul. 1996, page 31, and to xe2x80x9cMicromechanic Membrane Switches on Siliconxe2x80x9d in K. E. Petersen, IBM J. RES. DEVELOP., Volume 23, No. 4, Jul. 1979, pages 376-385.
The expression microrelay switch, for the purposes of this invention, relate to a switching device which has at least one microrelay cell. As described in detail further below, the microrelay switch may, however, be a complex system comprising a number of individual microrelay cells, with each individual cell having (at least) one moving contact piece.
In comparison to the described conventional component combinations, the switching device according to the invention thus offers a considerable reduction in weight and physical volume. The geometric flexibility of the overall arrangement is also improved, since the microrelay switch can be installed in widely differing manners, is at the same time particularly robust and insensitive to temperature fluctuations, shocks or the like and, in the case of a relatively large arrangement having a large number of microrelay cells, also provides major design freedom, since the conductor tracks between the microrelay cells can be formed as required.
A further primary advantage of the novel electrical switching device is the very rapid response of the microrelay switch. Due to the very much lower inertia of the moving masses, this represents a fundamental advantage in comparison to solutions using conventional contactors and relays. This is still true without any restriction even in the case of relatively complex microrelay switches having a greater number of microrelay cells, since the response time in this case is essentially the same as that with a single cell.
Furthermore, typical switching ratings and the power consumption of microrelay switches in the quiescent state are considerably reduced in comparison to conventional relays and contactors, and thus contribute to power saving and to reducing thermal problems, in particular in relatively large systems. Furthermore, the switching device according to the invention can also be used in combination with and for integration with other semiconductor-technology devices, in particular transistors and integrated circuits, since there are considerable corresponding features and overlapping areas in the production methods. The weight, volume and cost can thus be reduced further.
The evaluation device is a preferably microelectronic circuit whose more detailed design is immediately obvious to a person skilled in the art with respect to the respectively required functions in the various embodiments of the invention. In this case, the evaluation device can be designed to produce a certain time delay, in particular also as a function of the magnitude of a measured overcurrent, in comparison to the physically fastest-possible response of the microrelay switch. Further details relating to this can be found in the description of the exemplary embodiments.
Since electrical switching devices or overcurrent protection have to cover a very wide range of current and voltage requirements, the microrelay switch can be produced using a largely unchanging standard technology with different layout geometries, that is to say different mask sets. Widely differing electrical specifications can be covered in this case with a high level of technological standardization of the production line.
However, since microrelay switches are invariably subject to certain limits in terms of their current and voltage load capacity during operation in the field of present-day technology, and also during disconnection, the invention relates to a combination with a further element which is designed specifically for switching off large overcurrents. This refers to a short-circuit current limiter, with this term in this case referring only to a frequent cause of such large overcurrents, but not being restricted to this. The term short-circuit current is thus largely synonymous to large overcurrents, which considerably exceed the capacity to be switched off by the microrelay switch.
One particularly simple version of a short-circuit current limiter is a conventional fuse link which interrupts a large overcurrent by the melting of an incandescent filament or of a conductor track, but does not operate repetitively, that is to say it must be replaced. Such a fuse link is blown by the overcurrent itself, and thus does not require any actuation by the evaluation device.
A further option, however, is electrically operable tripping of the short-circuit current limiter. In this case, it is possible, but not essential, for the actuation of the short-circuit current limiter to use a further actuation signal (referred to as a second actuation signal in the following text) from the same evaluation device which also actuates the microrelay switch. By way of an example, an electrically tripped power breaker can be used for this purpose. However, it is also possible to use an entirely conventional power breaker, for example with an electromagnetic drive supplied by a short-circuit current.
A further version of the invention is for a PTC thermistor to be used as a short-circuit current limiter, or in addition to another switch, in a short-circuit current limiter. Such a PTC thermistor is defined by its electrical resistance having a positive temperature coefficient, which is sufficient that the heating of the thermistor at very large overcurrents causes a sufficiently severe rise in the resistance value to limit the overcurrent to values which can be switched off by the microrelay switch. This is the explanation of the term short-circuit current limiter, which thus covers both components which completely interrupt the short-circuit current and components which limit it to values which can be switched off by the microrelay switch.
Preferable PTC materials include PTC polymers, which generally consist of a polymer matrix with a filling material which is distributed in it and promotes electrical conductivity. The filling material may be, for example, metal particles, carbides, borides, nitrides, short carbon fibers, conductive polymer particles or else carbon black. Limiting can in this case can be carried out by means of a resistance rise by a factor of at least 2.5 in a temperature interval of 14 K, a factor of 6 in a temperature interval of 30 K, or a factor 10 in a temperature interval of 100 K, satisfying a criterion for definition. Preferable factors in this case are 5, 20 and 100, or 7.5, 100 and 1000, in the respective temperature intervals.
According to a further version of the invention, the switching device furthermore contains a fault current protection function. For this purpose, the current through two current paths is either detected or compared by means of two current sensors, with the result of this evaluation being used appropriately to open at least one microrelay switch in one of the two current paths, or a current sensor which is referred to here as a total current sensor can also be used to detect and evaluate a total current through two adjacent current paths, in order to open the microrelay switch. This is intended to mean a current sensor which, on the basis of the spatial conditions, detects the total (calculated for fault current detection) of both currents, taking into account their directions. For example, two conductor tracks as the current paths may carry the currents to be compared in mutually opposite directions, with the total current sensor detecting the total current, that is to say a zero current if the magnitudes of the currents are the same. Accordingly, the signal from the total current sensor may be compared only with a correspondingly small threshold value by the, evaluation device, in order to actuate the microrelay switch.
The fault current protection function can also, of course, relate to more than two current paths, for example with four current sensors for a three-phase current with a neutral line.
The current sensors mentioned several times in this description need not necessarily be part of the switching device according to the invention. In the simplest case, they may be conventional current sensors, for example induction coils. However, the invention preferably relates to Hall sensors, which can be produced with very small verification limits, for example with verification limits of about 1 mA in comparison to conventional verification limits of approximately 10 mA or more. Hall sensors can furthermore be produced as semiconductor elements to be very much smaller, lighter and also cheaper than conventional induction coils.
On the basis of what has been said above, the term microrelay switch may mean both an individual microrelay and a circuit comprising a relatively large number of microrelay cells. This should be understood in particular as meaning that, in any case in the present-day development standard, microrelays do not have an unrestricted current-carrying capacity and withstand voltage, and have only limited capabilities to switch off relatively high power levels. When the particular applications require load capacities beyond the given limits, of an individual microrelay cell, it is possible to use, according to the invention, voltage-dividing series circuits comprising two or more microrelay cells, and/or current-dividing parallel circuits. When combined, these are switch panels, namely voltage-dividing series circuits or parallel circuits which have a current-dividing effect in each stage of the series circuit.
However, in this context, it must be remembered that the technological limits in the present-day situation are subject to continual changes. Further improvements can be predicted with respect to the voltage and load capacities when switched on, and also with respect to the disconnection capacity. A joint research project between the manufacturer Bosch and a group at Bremen University is working on the development of microrelays with a maximum switching voltage of 24 V, and a maximum switching current of 25 A. It can thus be assumed that, in future, it will be possible, to satisfy even applications with somewhat more stringent requirements just by individual microrelays. This relates in particular to the current-carrying capacity, and it should then be possible to achieve the required withstand voltage by appropriate series connection.
Reference should now be made once again to an already mentioned advantage of the technologies that are typical for microrelays. For matching to a specific electrical configuration, a standardized microrelay cell based on a fixed standard technology can be designed for different parameter magnitudes by parallel and/or series connection. This can be done just by changing the layout geometry, for example by using a different mask set. The rest of the production process can remain virtually unchanged. This exploits the likewise already mentioned advantage of the very high response speed without scaling directly into the overall circuit. It is thus possible to produce extremely fast-response switching devices, in particular, with stringent specification values, in comparison to equivalent conventional contactors.
In addition to the integration of a switch panel having a number of microrelay cells, other integration versions are also of importance to the invention. The microrelay switch or switches, the evaluation device and, possibly, also the Hall sensor or sensors can, on the one hand, each be in the form of semiconductor chips and can be mounted on a common circuit board. This itself results in considerable advantages since the mounting technologies are identical or similar, and the combined components have a small physical size and low weight. As an example of a form of a Hall sensor which is highly related to microelectronics, reference is made here to the xe2x80x9cCylindrical Hall Devicexe2x80x9d by H. Blanchard, L. Chiesi, R. Racz and R. S. Popovic, Proceedings IEDM 96, pages 541-544, IEEE 1996.
When combined with a Hall sensor, it may, on the other hand, be advantageous to dispense, with complete integration, since it then possible to design a more highly standardized switching device using Hall sensors of different design depending on the application, for respective response values.
It is, of course, also possible for different components to be combined with one another on one chip. For example, the evaluation device and the microrelay switch or switches may be integrated. With suitable technology, this also relates to the Hall sensor or sensors. If, on the other hand, only the evaluation device and the Hall sensor or sensors are integrated, while the microrelay switch or switches, is or are in the form of a separate chip or chips, this allows the combination of a standardized chip with an evaluation device and a Hall sensor or Hall sensors with different, electrically differently designed, microrelay switches. Furthermore, temperature sensors, timer circuits and other electronic devices can also be combined and integrated.
In addition, this also relates to an electronic response monitoring device, which can likewise be combined or integrated. Such a response monitoring device registers the response of the electrical switching device and may, for example, be used to inhibit the switching device for a certain amount of time after it has tripped. Furthermore a defined test can be carried out after tripping, for example by means of a short switched-on state, in which a check is carried out to determine whether the fault state that was responsible for the previous tripping is still present. Automatic reconnection is thus possible, for example, following a temporary current surge. Interfaces to external control devices can also be provided.
In addition, timer circuits can also be combined or integrated, for example allowing use as an automatic time switch for lighting applications, for example stairwells.
Finally of course, devices for indicating the response to an overcurrent or a fault current in a visual or audible form are also possible.
One preferred application of the invention is a switching and protection system for an electric motor. The exemplary embodiments which are described in detail in the following text also relate to this, in which case, disclosed features may also be significant to the invention in other combinations. In the figures:
FIG. 1 shows a conventional electric motor switching and protection system;
FIG. 2 shows an electric motor switching and protection system according to the invention, having a switching device according to the invention, as a first exemplary embodiment;
FIG. 3 shows an illustration of a detail from FIG. 2, with details of the microrelay switch;
FIG. 4 shows an alternative to FIG. 2, as a second exemplary embodiment;
FIG. 5 shows a further alternative to FIGS. 2 and 4, as a third exemplary embodiment;
FIG. 6 shows a timing diagram to explain the second exemplary embodiment from FIG. 4; and
FIG. 7 shows a further timing diagram to explain the third exemplary embodiment from FIG. 5.